Python - find（）函数的代价 [英] Python - Cost of find() function

问题描述

我正在寻找一个有效的方法来检查一个短的字符串是否在一个长的字符串。我在这个线程上看到了一些建议：
Python有效的方法来检查是否非常大的字符串包含一个子字符串

但是，我没有看到使用find（）那里。使用find（）函数是否昂贵？什么是时间复杂性？

我看过维基页面，但没有找到find（）那里。
https://wiki.python.org/moin/TimeComplexity

解决方案

快速浏览源代码，看起来像 str.find 调用 stringlib_find_slice from 这里最终会调用 fastsearch 。实际的算法解释这里 - 与python伪代码（我从阅读评论）。看起来像在最坏的情况下执行O（N * M）（与天真的方法一样），但可以做O（N / M）在某些情况下（其中N和M分别是字符串和子字符串的长度）和O（N）在常见的情况下是这样的：。

^{1 （不要引用我的话 - 我只是剔除文档）}

I am looking for an effective way to check if a short string is in a long string. I saw some suggestions on this thread: Python efficient way to check if very large string contains a substring

However, I didn't see the use of find() there. Is it expensive to use find() function? What is the time complexity?

I've looked at the Wiki page but didn't find find() there. https://wiki.python.org/moin/TimeComplexity

解决方案

Quickly perusing the source, it appears that str.find calls stringlib_find_slice from here which eventually calls fastsearch. The actual algorithm is explained here -- with python pseudo-code (which I gleaned from reading the comments).

It looks like the implementation is in worst case O(N*M) (The same as a naive approach), but can do O(N/M) in some cases (where N and M are the lengths of the string and substring respectively), and O(N) in frequent cases¹.

^{1(don't quote me on it -- I only skimmed the document)}

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